1. Field of the Invention
The present invention relates to a member for a semiconductor device, in which a conductor layer consisting mainly of copper is bonded to an aluminum nitride substrate material, a method of manufacturing such a member for a semiconductor device, and a semiconductor device which employs such a member.
2. Description of the Prior Art
Conventionally, alumina (Al.sub.2 O.sub.3) has widely been used as an insulating substrate material for a semiconductor package, and a member in which a metallized circuit consisting mainly of tungsten is formed on the insulating substrate material in a multilayer structure has been used as a circuit board for a semiconductor IC. Alumina is superior in electrical insulation and mechanical strength, but its thermal conductivity is as small as approximately 17 W/m.multidot.K and its heat dissipation property is inferior. Alumina is, therefore, inappropriate to a circuit board on which to mount a high-capacity semiconductor IC.
In contrast, aluminum nitride (AlN) has recently been spotlighted as a substrate material for a circuit board because of its electrical insulation and mechanical strength approximately equivalent to those of alumina, its light weight and its high thermal conductivity exceeding 100 W/m.multidot.K. In addition, aluminum nitride exhibits a mean coefficient of thermal expansion as small as 5.5.times.10.sup.-6 .degree. C. in the temperature range of from the room temperature to the silver-soldering temperature (approximately 800.degree. C.), so that aluminum nitride exhibits superior bondability and compatibility with an Si semiconductor chip (which has a coefficient of thermal expansion of 4.0.times.10.sup.-6 /.degree. C.). However, aluminum nitride is poor in bondability with Kovar (having a coefficient of thermal expansion of 10.times.10.sup.-6 /.degree. C.) and a 42 alloy (having a coefficient of thermal expansion of 11.times.10.sup.-6 /.degree. C.) which are used for a package material or a through lead to a circuit board.
It is generally known that various intervening layers are formed between nitride ceramic and metal so that the nitride ceramic and the metal are bonded to each other. For example, Japanese Patent Publication No. 2-34908 (1990) states that a layer made of a low-elastic-modulus metal and/or a metal having malleability and ductility, a layer made of a brittle material and a layer made of a material having a low coefficient of thermal expansion are formed as intervening layers in multilayer in this order from the ceramic side. However, bonding which uses these kinds of multiple intervening layers easily lowers the thermal conductivity by the multiple intervening layers provided for bonding purpose, and the application of such bonding to an aluminum nitride heat sink board is limited practically.
For this reason, it has been a common practice to form a metallizing layer of W, Mo or the like on the surface of an aluminum nitride substrate material and bond the aluminum nitride substrate material to a metal member such as a lead frame or a package by silver-soldering via the metallizing layer. For example, Japanese Patent Laid-Open No. 63-289950 (1988) discloses the art of forming a W metallizing layer on an aluminum nitride substrate material and bonding a lead frame made of oxygen free copper having a high heat conductivity and a high thermal shock absorbing property (refer to FIGS. 1 and 2 of Japanese Patent Laid-Open No. 63-289950 to the W metallizing layer by silver-soldering. In this art, if necessary, an Ni layer for improving the wettability is formed on each of the W metallizing layer and the oxygen free copper lead frame, and both are bonded to each other by silver-soldering.
In accordance with the aforesaid method of bonding the oxygen free copper lead frame to the aluminum nitride substrate material via the metallizing layer, thermal stress due to heating during silver-soldering is greatly reduced compared to ordinary lead frames using Kovar or the like, so that the bonding strength which lowers in the case of Kovar does not lower. However, the aforesaid method involves the problem that the shape of the lead frame is difficult to maintain because oxygen free copper is a soft material. In addition, if a copper-based member is joined to an aluminum nitride substrate material via a silver-solder layer in the above-described manner, a large thermal stress action due to the silver-soldering occurs owing to the difference in thermal expansion between the silver-solder and the aluminum nitride, so that breakage or deformation, such as cracking or warp, easily occurs in the aluminum nitride substrate material after the cooling. This leads to the problem that a special expensive silver-solder material which is silver-rich and soft needs to be employed to lower the cooling stress, or strict control for a small-amount region is needed to make the silver-solder layer thinner.
Under the circumstances, investigations have been made into various methods of bonding a metal member which is a conductor to an aluminum nitride substrate material without forming an intervening layer of solder material such as silver-solder. One method is a so-called DBC (direct bonding copper) method which does not use a W metallizing layer nor a solder layer to bond copper as a metal member to an aluminum nitride substrate material.
For example, Japanese Patent Laid-Open No. 59-40404 (1984) discloses a method which includes the steps of forming on the surface of an aluminum nitride substrate material either a layer of an oxide of the aluminum nitride substrate material itself or a binding layer made of an oxide of aluminum, a rare earth element or an alkaline earth element which are used as sintering aids for the preparation of a sintered body of aluminum nitride, preparing, as a counterpart to be bonded to the aluminum nitride substrate material, a metal material which contains a little amount of a binder of such oxide (which may contain oxygen alone) or has such layers formed on its surface in advance, and directly bonding the aluminum nitride substrate material and the metal material by using the affinity between the binding layers on these materials. For example, if the metal material is made of copper, it is bonded to the aluminum nitride substrate material having the oxide layer thereon, using the copper oxide formed on its surface, by subjecting the material to heat treatment in the temperature range of from the eutectic temperature of the copper oxide and copper to the melting point of copper.
A similar method is disclosed in Japanese Patent Laid-Open No. 60-32343 (1985). This method is a bonding method in which a thin copper-alloy eutectic layer containing an active metal (such as Ti, Zr or Hf) is intervened between an aluminum nitride substrate material and a copper beat sink board. Another DBC method is described in "Electronics Ceramics", the November issue, 1988, pp. 17 to 21. In this method, a thin aluminum oxide layer of up to several microns is first formed on the surface of an aluminum nitride substrate material and then copper is bonded to the thin aluminum oxide layer via a Cu.sub.2 O--Cu eutectic layer.
However, in any of the above-described methods of bonding copper to aluminum nitride by using an eutectic region of a copper oxide and copper, the variation of the bonding strength easily becomes great unless the thickness of the oxide layer on the aluminum nitride is controlled within a narrow range, as illustrated in FIG. 4 of the above-cited report of "Electronic Ceramics". In addition, in these methods, since an intervening layer made of aluminum oxide and a copper oxide eutectic component formed between an aluminum nitride substrate material and a copper member is thin, breakage or deformation such as cracking or warp easily occurs in the substrate material owing to the difference in thermal expansion between copper and aluminum nitride. In addition, it is necessary to create a special oxygen partial-pressure atmosphere for eutectic bonding of copper and copper oxide at around 1,000.degree. C. Since the surface of the copper member is oxidized by the special oxygen partial-pressure atmosphere, an extra step such as surface polishing is needed before the copper member is subjected to solder-bonding. When the copper member is mounted on the aluminum nitride substrate material, it is necessary to carry out the time-consuming step of performing positioning for defining a non-mounting portion, and forming with good reproducibility the boundary between the copper member and a fuse-contact portion on which to mount the copper member.
In the method using an active metal as described in Japanese Patent Laid-Open No. 60-32343 (1985), an expensive active metal solder material is needed, and a high vacuum of not greater than 10-4 Torr is needed during soldering. In many cases of soldering in nitrogen gas, it is also necessary to prepare a special metal soldering material containing, e.g., a large amount of Ti in advance. Furthermore, if such an active metal solder material is employed, voids are easily produced in the interface between aluminum nitride and the active metal solder material, so that cracking easily occurs therein. Thermal resistance may also increase because of the presence of the soldering material.